The present invention relates to the making of a sample or measuring chamber or cuvette to be used in photometric and/or spectrometric or spectrographic measuring equipment. Devices of the kind to which the invention pertains need sealed chambers which hold a gas in a well defined concentration in a tightly sealed manner so that the gas will not be diluted or diffused to the exterior of the chamber. Such a sample or measuring chamber is usually made of metal and has a particular length.
Since the purpose of the chamber is to subject a specific amount of gas to radiation for purposes of absorption of some of the radiation by the gas, it is necessary that the chamber have front and rear sides provided with windows. For example, calcium fluoride CaF.sub.2 is provided for such a window, i.e. as the window pane. Of course, the windows must be gas-tightly sealed vis-a-vis the metal body of which the chamber is made. The measuring effect results, of course, from the absorption of light passing through this chamber and through the windows, the absorption being carried out by the enclosed gas. Since the gas concentration and quantity are to be constant, there is a correspondingly high degree of constancy of absorption.
A variety of technologies are known for the construction of such measuring chambers (see for example U.S. Pat. No. 5,003,175). The state of the art is given in a data sheet and user's manual by applicant's assignee. It lists as known a device traded under the designation 42/22-22-0.9.79 for use in a gas analyzer called "Radas 1G" by the Isami Corporation. And here, on page 40, a calibrating chamber is described as including quartz and transmission glasses. A gas is permanently enclosed, i.e. supposedly trapped and confined after the chamber has been closed through melting. However, such a melting procedure for purposes of closing the chamber is not suitable if the chamber contains hydrocarbons or Nox gases. These gases change when subjected to temperatures as high as required for melting quartz or glass. On the other hand, quartz and glass are not suitable for passing medium range infrared radiation instead of visible light. This is so because infrared radiation is already absorbed at wave lengths of about 4.5 micrometers, that is, in a band of about 2.5 micrometers. For that reason also, a sapphire is usable only to a very limited extent. Its absorption edge is situated at about 5.5 micrometers and will therefore be insufficiently capable of acquiring information of the absorption spectrum of SO.sub.2 and many hydrocarbons.
German printed Patent Application 30 10 516 describes a measuring chamber for optical gas analyzers wherein the radiation-permeable windows are connected by means of a glass solder used as a brazing material providing the connection of the windows to an intermediate frame. The thus prepared windows are then gas-tightly connected to the measuring chamber proper through regular solder or brazing. On the other hand, German printed Patent Application 27 20 636 describes radiation-permeable windows of calcium or barium fluoride.
Owing to its transparency to infrared radiation, particularly in the median infrared range up to a wave length of about 8 micrometers, fluorite is occasionally used as end windows in a measuring chamber. This arrangement does indeed permit ascertaining the absorption sprectra of most gases of interest as far as measuring technology and data acquisition is concerned. Also, this particular material is suitable because it is not hygroscopic and has no catalytic properties vis-a-vis the usual filling contained in the chamber.